Spacecraft are subjected to a wide range of thermal environments during their service. For example, in use, one side of a spacecraft may face in a direction away from the sun, while another side faces towards the sun. Thermal control is desirable because heat is radiated into space, which cools the spacecraft, but the spacecraft can simultaneously be heated intensively in direct sunlight. Active and passive temperature control techniques are therefore generally used to maintain the interior temperature of the spacecraft, which generally contains persons or sensitive instruments, within acceptable operating limits. Active temperature control may involve machinery or electrical devices, such as electrical heaters and/or coolers. In contrast, passive temperature controls are techniques that do not involve machinery or electrical devices, but include thermal control coatings or structural designs.
One known passive temperature control technique includes use of surface coatings, typically termed thermal control coatings or thermal control paints, on the external surface of the spacecraft. A thermal control coating may be defined as a surface whose thermo-optical properties may be designed in order to achieve a desired surface temperature when subjected to a known solar flux or other source of radiation. A white thermal control paint, for example, has a low solar absorbance, while a black paint has a high solar absorbance. Selective application of such paints to various elements of the spacecraft exterior greatly aids in controlling its temperature.
It is generally recognised that the temperature at the surface of a thermal control coating is dependent on the ratio of the coating's optical absorption to thermal emissivity, which is naturally greatly affected by the material(s) of the coating. It is generally accepted that the beginning of life (BOL) optical properties required for a thermal control coating suitable for use for coating spacecraft, are an optical absorbance (also known as solar absorbance or solar absorptance) (αs) of no greater than 0.20, meaning that less than 20% of the solar radiation impinging on the spacecraft external surface is allowed to be absorbed through to the interior; and a thermal emissivity (εN) of no less than 0.80, meaning that at least 80% of the internal heat generated is emitted to the cold vacuum of space.
In addition to passive temperature control, it is desirable for a coating applied to the surface of spacecraft to dissipate electrostatic charges (i.e. to be capable of electrostatic dissipation, ESD) that may develop along the external surface of the spacecraft. Otherwise, the electrostatic charges may accumulate and cause arcing and possible damage to, or interference with, sensitive electronic equipment on or in the spacecraft. In order to dissipate electrostatic charge, the coating must have at least some electrical conductivity. It is generally accepted that it is desirable that coatings capable of electrostatic dissipation (ESD) have volume and surface resistivities of less than about 109 Ωm and 109 Ω/sq respectively.
In addition to thermal control and ESD, a coating suitable for use on spacecraft and spacecraft components should exhibit additional characteristics for spacecraft applications. For example, the coating should be stable during long-term service in a space environment, including the ability to survive micrometeoroid impacts and high levels of radiation exposure. The coating should be moderately tough and flexible so that it does not crack and flake away as it is flexed due to mechanical or thermal strains.
A number of white, electrostatic-dissipative coatings are known for spacecraft use. One of the most well-known coatings is Z-93, developed by the Illinois Institute of Technology Research Institute (IITRI), 10 West 35th Street Chicago, Ill. 60616. Z-93 is a white paint comprising 41.52% zinc oxide (ZnO), 32.05% potassium silicate, and 26.43% de-ionised water. This paint was developed prior to 1964 and has been used as a stable white thermal control coating formulation. Its optical properties of αs=0.18±0.03 and εN=0.9±0.05 (G. R. Smolak and N. J Stevens. Report on the flight performance of the Z-93 white paint used in the SERT 2 thermal control system. Technical report, National Aeronautics and Space Administration (NASA), 1971) have rendered it suitable for coating spacecraft.
Although Z-93 has been widely used to coat spacecraft in the past, Z-93 is not without its disadvantages. For example, Z-93 has been reported as being porous, thermochromic, and having less than optimal electrostatic dissipation. The porosity of the Z-93 coating was discussed by William F. Carroll during the 1964 internal NASA conference on spacecraft developments (William F. Carroll. Coating development and environmental effects. In NASA Conference Proceedings on Spacecraft Coating Developments, pages 1-9, May 1964), wherein he stated with regard to Z-93 that “The ZnO-Potassium silicate coating is the most stable formulation developed, but, like all non-vitreous inorganic coatings, has adverse physical properties which limit its use. The coating is porous and therefore easily soiled and difficult to reclean. Therefore, use should be limited to applications where surfaces can be easily protected from contamination or where requirements for maximum stability justify extreme precautions for prevention of contamination”.
The thermochromic nature of Z-93 is also well recognised, meaning that at high temperatures of greater than about 300° C., the colour of Z-93 changes from white to yellow, as is the case with all zinc oxide based surface treatments. Thermochromism in such coatings can be disadvantageous as an immediate increase in αs will be observed upon exposure to high temperature, which is undesirable. Additionally, temperature fluctuations in service will result in further instability of αs. In addition, the electrical resistivity (ESD) for Z-93 has been reported as 9.26×1015 to 3.65×1016 Ωm (see e.g. page 2-14 of Deshpande & Harada. Development of Tailorable Electrically Conductive Thermal Control Material Systems, National Aeronautics and Space Administration (NASA), June 1998), which is greater than the desired maximum 109 Ωm. In summary, although Z-93 has been used quite successfully as a white thermal control coating on spacecraft, it does have disadvantages.
Variants of Z-93 also exist, for example the white thermal control paint AZ-93 developed by AZ Technology. The product description provided for AZ-93 on the AZ Technology website www.aztechnology.com indicates that, similar to Z-93 discussed above, the solar absorbance and thermal emissivity again render AZ-93 paint suitable for coating spacecraft (αs=0.15±0.01 @≥5 mil (127 micron) thickness; εN=0.91±0.02). However, the same disadvantages remain with the AZ-93 coating as discussed above for Z-93 in terms of being porous, thermochromic, and having less than optimal ESD.
It is therefore an object of embodiments of the invention to overcome or mitigate one or more of the disadvantages associated with conventional coatings such as Z-93 or AZ-93.
It is also an object of embodiments of the invention to provide a coating, especially a white thermal control coating, which is operable and stable in a space environment. It is a further object of embodiments of the invention to provide white thermal control coatings which have an optical absorbance (αs) of no greater than 0.20 at ≈100 μm thickness and a thermal emissivity (εN) of no less than 0.80 at temperatures up to 500° C., which are also capable of electrostatic dissipation, are less porous than conventional coatings, and which are non-thermochromic at temperatures up to or even greater than 500° C.